Effect of currents and tides on planing hulls

Discussion in 'All Things Boats & Boating' started by valvebounce, Oct 10, 2013.

  1. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Hi,I'm pretty new to seamanship,I have a 13ft speedboat which I have converted to a pleasure/fishing boat.The hull is fibreglass,the bow rake continues 1/3,and then is more or less flat to the stern,with a more or less vertical transom,which I have replaced to suit a longshaft outboard.(18hp evinrude fast twin)
    What I would like to know is-does the current and tide effect a planing hull as much as a displacement hull?Would it lose power in estuaries that have a strong current?I have noticed that inflatable ribs seem to skip over the surface without the current effecting them too much.
    I bought this boat as a project,and have never seen it in the water.It didn't have a deck,I replaced the stringers and upper/lower futtocks and am in the process of fitting one.
    Any help will be greatly appreciated
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    G'day VB, how's things in "Blighty" ? The boat's speed through the water is pretty well independent of current and tide, but speed over the ground is affected, of course. The "ground" in this instance being the sea-bed. I suppose if your boat planed at 10 knots, and the current was 10 knots, you could have a long conversation with someone on the riverbank, if you were travelling directly into the current. I recall during a big flood in the Brisbane River in 1974 seeing a big tugboat barely able to make headway up-river, to come to the rescue of a big ship that was in danger of breaking loose, such was the speed of the flow. Most planing boats go fast enough to less affected than a slow putt-putt boat that might be going backwards in an adverse run.
     
  3. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    G'Day mr E,thanks for your reply.Blighty ain't so hot,I have been back here 50yrs this month,and rued the day we left Melbourne ever since.
    I understand what you mean about "over the ground"
    I have taken this boat back to it's original fibreglass hull,the only timber that is original are the embedded stringers,which were intact.I have got the permission to float it on Rudyard lake,providing I don't start the engine because of the pollution.I have re-designed it somewhat,so I need to float it fully equipped to get the bow to stern trim level.I am assuming that getting the waterline on the boat level with the water is the way to go.I am taking some concrete building blocks aboard to shuffle fore and aft to get the balance as a temporary measure,then noting the weight and position of them before I finish the internals.I am building a locker under the bow bonnet,and raising it above deck level to keep things dry.With a bit of luck what I put in it will counterbalance the engine and fuel tanks at the stern.( I just hope it floats Haha)
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I think in a small boat like this, you can't get too pedantic about where the fixed weights are, fore and aft, 'cos the non-fixed weights (the people on board) are such a high % of the total weight, they will dominate how the boat trims, particularly at rest, on the plane, trim is a little less influenced by shifts in weights.
     
  5. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for the info,I haven't decided on the seat positions yet,it's tiller steer,so one will need to be at the stern.
    My idea is to get on the plane at the slowest speed possible for economy reasons,I have a set of Doel fins for the engine,and have set the new transom height as high as is practical.V
     
  6. Ike
    Joined: Apr 2006
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    Ike Senior Member

    13 foot with an 18 Hp and it's tiller steered? You'll probably have water come over the transom. I suggest strongly you put a steering wheel or stick steering up forward and put your seats there to balance out the weight of the engine on the transom. As was said before the largest % of weight is the people. So you need to move them closer to the longitudinal center of the boat to keep it trimmed flatter. I would aso suggest you keep it to 2 people and a dog. 3 people may be too much.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    The tiller steer issue will depend on the beam aft, if it's over say, 4.5 feet, chine-to-chine at the transom it might be OK, but in a skinny boat it won't work. And I guess the weight of the tillerman is not irrelevant either.
     
  8. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for your interest Ike,I have reduced the length of the bow hood cover
    to give more deck space,and also to enable me to move the seating weight forward.I have changed the transom to take a longshaft engine.The boat is not of a flimsy design,and I have added a deck out of 9mm exterior ply.Like you say,I will need to pay attention to the balance.There will probably be only two aboard most of the time,and it will be lightly loaded,mainly fishing gear.V.
     

  9. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    G'day mr E,The boat is 4ft-4ins at the deck level on the transom(internal)
    it is more or less flat bottomed from 2/3 the way down from the bow.There are no chines aft,only at the bow,and the ones at the bow are incorporated into the hull shape,and don't have much definition.The widest point across is
    4ft-7ins internally at deck level.I don't have any history of the boat,but I suspect it has been a ship to shore launch on a bigger vessel.There were a couple of eyebolts just above the waterline on the starboard side,which I suspect were used to secure the boat onto another larger vessel.The boat,although not heavy as a real solid feel to it.The new cross futtocks don't touch the hull,they are let in to the new stringers,and the deck is sitting on them.I have not changed the longitudal balance much,but I will need to consider it for a decent planing angle.
     
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